1. Field of the Invention
The present invention relates to an electromagnetic control valve for a variable capacity compressor and, more particularly, to an electromagnetic control valve for a variable capacity compressor employed in a cooling unit for vehicles.
2. Related Art
Japanese Patent Application Number 8-109797, filed by the present applicants, discloses an electromagnetic control valve for a variable capacity compressor used in a cooling unit for vehicles (hereinafter referred to as "prior art A"). This electromagnetic control valve can reduce the load on the engine at the time of starting a vehicle, and it can be small in size. Furthermore, the valve characteristics of the electromagnetic control valve can readily be changed.
FIG. 5 is a schematic view of a capacity control mechanism of a variable capacity compressor in which the above electromagnetic control valve is employed.
A variable capacity compressor 20 has an electromagnetic capacity control valve 1-3 shown in FIG. 6 secured to its attachment concave portion 40. A plurality of cylinders 25 are provided inside a head portion 41 which communicates with a crankcase 21 of the compressor 20, and a piston 26 is slidably provided inside each cylinder 25. A driving shaft 27 is rotatably provided in an area extending from the crankcase 21 to the head portion 41. The driving shaft 27 is driven by an engine (not shown) via a pulley 34 and a belt 35 provided outside the crankcase 21.
The driving shaft 27 is provided with a wobble plate 29 which can vary the inclination of itself. The wobble plate 29 is linked to the pistons 26 through piston rods 24. The wobble plate 29, which is inclined as shown in FIG. 5, is rotated by the driving shaft 27, so that the piston rods 24 and the pistons 26 can move back and forth. By doing so, the attachment angle of the wobble plate 29 can be automatically adjusted depending on the differential pressure between a control chamber pressure Pc in the crankcase 21 and a suction side pressure Ps in the cylinders 25, while the stroke width of the pistons 26 can be changed depending on the inclination of the wobble plate 29.
Each cylinder 25 is provided with a suction opening S and a discharge opening D, and connected, via passages d and s, to a condenser 31, an evaporator 32, and an expansion valve 33. The electromagnetic control valve 1-3 communicates with a crank chamber C (or a control chamber C) in the crankcase 21 via a refrigerant passage 36, with the suction opening S via a refrigerant passage 37, and with the discharge opening D via a refrigerant passage 38.
FIG. 6 is a sectional view of the electromagnetic control valve of the prior art A when not energized.
The electromagnetic control valve 1-3 includes: (a) a valve member 8a which is provided in a valve chamber 18 formed by an upper main body 7 and a valve main body 17, and which repeatedly comes into contact with and depart from a valve seat 17d formed in a valve opening 17k situated on the way to a Pc pressure introducing opening 17h leading to the crank chamber C in the crankcase 21; (b) a Pd pressure introduces opening 17i which is formed on the opposite side of the Pc pressure introducing opening 17h from the valve chamber 18, and which leads to the discharge-side refrigerant passage 38 of the compressor 20; (c) a valve guide 17e extending from the Pd pressure introducing opening 17i to the Pc pressure introducing opening 17h; (d) a valve stem 8b which is integrally formed with the valve member 8a, and which is movable in the vertical direction inside the valve guide 17e; (e) a pushing unit 9 for pushing the valve member 8a in the valve opening direction; (f) a plunger 5 situated above the upper main body 7 which is vertically movable by virtue of the suction power of an electromagnetic coil 15, and which pushes the valve member 8a in the valve closing direction via a connecting rod 6 which is vertically movable in a through opening 7b of the upper main body 7; (g) a pressure equalizing opening 17q formed through the valve main body 17 and extending from the valve chamber 18 to the Pd pressure introducing opening 17i; (h) pressure bellows 11 formed below the Pd pressure introducing opening 17i and situated in a space 17g which communicates with the suction side S of the compressor 20; and (i) a corrective pin 10 which comes into point-contact with the lower surface of the valve stem 8b at its upper end and with the pressure bellows 11 at its lower end, and which is movable in a guide opening 17f extending from the Pd pressure introducing opening 17i to the space 17g.
With the suction pressure Ps of the piston cylinders 25 in FIG. 5, the pressure bellows 11 control the opening angle of the valve member 8a, which is situated in the refrigerant passage extending from the refrigerant passage (a discharge pressure supply passage) 38 to the crankcase 21.
A casing 2, a coil guide 3, the plunger 5, the connecting rod 6, the upper main body 7, the electromagnetic coil 15, and a plunger tube 16, integrally constitute an electromagnetic actuator. The opening angle of the valve is controlled depending on the current supplied to the electromagnetic coil by the electromagnetic actuator.
Japanese Patent Application Laid-Open No. 5-99136 also discloses an electromagnetic control valve for a variable capacity compressor (hereinafter referred to as "prior art B"). This electromagnetic control valve is provided with a first valve in the refrigerant passage extending from the high-pressure discharge side (Pd) to the crankcase side (Pc) of the compressor, and a second valve in the refrigerant passage extending from the crankcase side (Pc) to the suction side (Ps) of the compressor. Thus, the pressure Ps will not be directly affected by the pressure Pd, and the pressure Pc will not become too high when the discharge capacity decreases as the discharge gas (Pd) is introduced into the crank chamber.
With the electromagnetic control valve of the prior art A, the variable capacity point of the compressor can be varied by changing the set point of the suction pressure Ps. Since the absolute valve of the pressure Ps needs to be obtained in such method, high reliability is required in the sealed structure of the pressure bellows and the like. This causes a problem that it is expensive to produce such sealed structure formed by beam welding or the like.
In each electromagnetic control valve of the prior art A and prior art B, the suction pressure Ps of the compressor ranges from 1.5 to 4 Kg f/m.sup.2 depending on the load, and it nears 10 kg f/m.sup.2 at the time of starting. To control as desired under a heavy load, large electromagnetic force is required to meet the high suction pressure, and therefore, the coil should be large in size.
Further problems with the electromagnetic control valve of the prior art are that large electromagnetic force is required in obtaining a wide range of pressure, and that the sealed structure makes the hysteresis undesirably high, resulting in unreliable control operations.